1. Field of the Invention
The invention relates to a magnetoresistive sensor using the magnetoresistance of ferromagnetic materials. A sensor of this kind is designed to detect magnetic fields. In the field of low frequencies (continuous frequencies at 100 Hz typically), the two noise sources that limit the resolution of this type of sensor are the resistance fluctuation noise or Johnson noise and the thermal drift noise. Furthermore, in this type of sensor, two different magnetoresistive effects can be used: anisotropic magnetoresistance and giant magnetoresistance.
2. Discussion of the Background
The anisotropic magnetoresistance effect results from the dependence of the resistivity of a ferromagnetic material on the angle between its magnetization and the direction of the current. Owing to the anisotropic character of this physical effect, two measurement geometries can be considered: longitudinal geometry and the transversal geometry (planar Hall effect). The French patents 92 15551, 95 05659 and 96 08385 describe planar Hall effect sensors, especially with respect to the reduction of thermal drift noise. The total exploitable effect is in the range of 1% of the resistance of the active zone. The Johnson noise is associated with the same resistance which is in the range of the resistance per square unit of the film in terms of planar Hall effect geometry.
The effect of giant magnetoresistance was discovered in 1988 (see M. Baibich et al., Physic Review Letters, 61, 2472 (1988)) and results from the spin dependence of the resistance of a magnetic structure that can have an arrangement of magnetizations that differs according to the external magnetic field. The total exploitable effect may be in the range of 10% of the resistance of the active zone. Furthermore, since the geometry of measurement of this effect is necessarily longitudinal, the active zone can be sized in such a way that its resistance attains several tens of times the resistance per square unit of the film, the voltage equivalent of the Johnson noise increasing as the square root of the resistance. It can therefore be seen that the use of the giant magnetoresistance effect, especially with respect to sensors using the planar Hall effect, has two main advantages: an increase in the amplitude of the useful signal that is certainly capable of attaining one order of magnitude as well as an increase in the signal-to-noise ratio which too can attain a factor of ten.
The invention proposes to use the technique of giant magnetoresistance to obtain a linearizing of the signal and reduce the noise of thermal drift.
The invention therefore relates to a magnetic detector comprising a first thin-layer element made of magnetic material with magnetic anisotropy in the plane of the thin layer possessing, in this plane, an easy magnetization axis, characterized in that it comprises a second thin-layer element parallel to the first element, this second element being made of magnetic material with magnetic anisotropy in the plane and having, in this plane, an easy magnetization axis parallel to that of the first element, the coercive field of the second element having a value different from that of the first element, the two elements having elongated and mutually parallel shapes perpendicular to their direction of easy magnetization in the absence of a magnetic field and the width of these elements being such that it obliges at least one of the elements to have its magnetization, when there is no external magnetic field, oriented along the length of the element.